In concrete wall, mine roof and rock wall support systems (hereinafter collectively generally referred to as “mine roof support systems”), it is known to embed longitudinally elongated rod-like reinforcing rebars or anchor bolts within drill holes ranging from 25 mm to 45 mm which have been drilled in the wall or rock face. Typically, a number of grout or two-part resin cartridges are pre-inserted into the bore hole ahead of the anchor bolt. The reinforcing anchor bolts each comprise a four to twelve foot length of steel which is threaded along its outermost proximal end. The bolts are inserted into the drill or bore hole, so that the threaded end projects outwardly beyond the wall or rock face, permitting the threaded coupling of a nut thereto. Once inserted into the drill hole, the bolt is spun or rotated about its longitudinal axis, while it is slowly inserted into the resin or cement cartridge, to assist in mixing the grout or resin, securing it in place.
To spin the rod, a nut or threaded fastener is coupled to the proximal end of the bolt, and which following setting of the resin, is tightened against the wall or rock face to consolidate forces, and prevent or control ground movement. Frequently, the torquing nut consists of a dome nut having a deformable end cover, threaded onto the anchor rod and used to rotate the bolt and thus aid in resin mixing. Conventional cast dome nuts typically are formed having a threaded socket which extends into a domed end portion. The domed end portion is formed with a thickness such that its engagement with the end tip of the bolt prevents further movement of the dome nut onto the bolt end under initial torque forces, with the result that the bolt rotates together with the turning of the dome nut. As the grout/resin sets, resistance to the rotation of the bolt increases. As the rotational torque forces applied to the nut exceed a critical minimum or threshold rotational torque force, the domed end portion of the nut splits apart by the contact pressure forces of the bolt end thereagainst, allowing the nut to be tightened along the bolt and against the rock face. Another torquing nut employed is a pin nut. This consists typically of a 1.125″ square nut threaded onto the end of the bolt threaded section and a small drill hole, typically 5/16″, is machined through both the nut and bolt. A pin such as a 5/16″ roll pin is inserted into the drill hole on the side of the nut. The pin nut prevents the advancement of the nut while it is mixing the two part resin cartridge. Once the resin has set up or hardened, the rotational torque forces applied the nut exceeds a critical minimum rotational force required to shear the roll pin, split pin or solid pin, thus allowing the nut to travel along the treaded section and apply load the rock face.
In certain mine roof support applications, it is desirable to provide an anchor rod or bolt which is constructed so as to yield, whereby the bolt is moved axially along the bore hole through set grout or resin, to assist in absorbing either static or dynamic ground forces. With such yieldable anchor bolts, forces generated by dilating rock are transferred to the anchor bolt via the tightened nut. As the rock forces reach a predetermined minimum yield force, the yieldable anchor bolt is partially drawn outwardly from the bore hole, moving axially through the anchoring resin, thereby absorbing the force from the surrounding rock and returning the system to a state of equilibrium. International Publication No. WO 02/02910 A2 to Gaudreau, published Jan. 10, 2002, describes one such yieldable cone bolt construction, used as a reinforcing rod in mine roof support systems. The cone bolt described in Gaudreau consists of a steel bar which has a conical wedge-shaped projection at its inner distal end, and which extends radially to a diameter of about 2.5 mm. A 2 to 2.5 cm long mixing tab is mounted to the end of the conical projection for use in assisting in the mixing of resin used in the initial securement of the cone bolt in the bore hole.
Conventional yieldable cone bolts suffer a disadvantage in that heretofore, their construction has been poorly suited to achieve even mixing of the anchoring resin. In particular, even where a mixing tongue or blade is provided, to ensure that the blade does not interfere with the axial movement of the bolt when yielding, it is necessary to manufacture the bolts so that the blade has a maximum diameter no larger than that of the enlarged cone projection, and which, in use, is typically about ⅔ the diameter of the bore hole. In particular, heretofore the radial diameter of mixing blades has been largely kept no larger than that of the enlarged conical projection where it may otherwise interfere with the yielding movement of the cone bolt upon the application of a minimum yield force thereto. As a result, (the narrower dimension of) the mixing blade relative to the bolt hole often results in the incomplete mixing of resin, particularly in the areas immediately adjacent to the bore hole walls. This in turn may result in incompletely mixed resin along the bore hole sidewalls, which could result in inconsistent, unpredictable or unreliable yielding of the anchored bolt.